Telescopic sights for rifles typically include eyepiece and objective lenses positioned at opposite ends of a tubular housing. Conventional variable optical power telescopic sights also include an erector lens positioned medially of the eyepiece and objective lenses and movable along a central longitudinal axis of the housing for adjustment of the optical power.
In these sights, an image focus adjustment compensating for variations in the distance from the marksman to the target is typically accomplished by longitudinal movement of either the eyepiece or objective lens. One prevalent image focus adjustment implementation uses a threaded mounting ring that is coupled to the housing for rotation about the eyepiece end or objective end of the housing. The marksman rotates the mounting ring to adjust the position of a focusing element of the eyepiece or the objective lens along the longitudinal axis of the housing.
This implementation is prone to image distortion caused by tilting of the objective or eyepiece lens during focus adjustment and makes it difficult to seal the ends of the housing to maintain within the telescopic sight a nitrogen gas charge necessary to prevent fogging and condensation on internal lens surfaces. This implementation also requires the marksman to stop viewing the target through the telescopic sight, move the rifle out of shooting position, and rotate the rifle to read markings on the outside of the housing or focusing ring which indicate the amount of focus adjustment. After viewing the focus ring adjustment indicator markings, the marksman then must reacquire the target.
A technique for varying the optical power of a telescopic sight is described in U.S. Pat. No. 3,058,391 of Leupold. This telescopic sight has an erector lens positioned between an objective lens and an eyepiece lens and adjustable along the longitudinal axis of the housing. It includes a field lens or collector lens located between the objective and the erector and movable in conjunction with and relative to movement of the power varying erector to compensate for focus effects resulting from movement of the erector. Movement of the collector and erector lenses to maintain focus during adjustment of the optical power of the telescopic sight is accomplished by turning a single adjustment ring connected to a cam sleeve mechanism within the interior of the sight housing.
U.S. Pat. No. 4,643,542 of Gibson describes a fixed optical power telescopic sight with a focus adjustment that is accomplished by longitudinal movement of an erector lens. In this telescopic sight, a focus knob mounted laterally on the outer side of the sight housing controls the erector movement. This telescopic sight has objective and eyepiece lenses that are fixedly mounted within the housing to form an air tight seal and maintain nitrogen gas charge within the housing. This design does not, however, provide adjustment of the optical power of the telescopic sight.
Because bullet trajectory, wind conditions, and distance to the target can vary depending upon shooting conditions, quality rifle scopes typically provide compensation for variations in these parameters by allowing the marksman to make small adjustments to the optical characteristics or the aiming of the sight relative to the rifle on which it is mounted. These adjustments are known as windage and holdover and are typically accomplished by lateral movement of the reticle within the telescopic sight, as shown in U.S. Pat. No. 3,058,391 of Leupold, or pivotal movement of lenses mounted to a pivot tube within the housing to divert the optical path of the observed light before it reaches the reticle, as shown in U.S. Pat. Nos. 3,297,389 and 4,408,842 of Gibson. In these designs, a marksman accomplishes adjustment of windage and holdover by turning a laterally protruding knob or screw that is operatively connected to the reticle or pivot tube. The range of adjustment for windage and holdover in these designs is limited by the space available within the housing for lateral movement of the reticle or pivotal movement of the pivot tube.